The present invention relates to estimating the viscosity of a liquid in a metering system, such as a liquid transported by a metering system on a diagnostic analyzer. In particular, the present invention relates to estimating viscosity by selected pressure measurements to more accurately determine an amount of fluid aspirated or dispensed.
In the diagnostic field, there is a trend for using smaller and smaller amounts of liquid in a diagnostic analysis or assay, such as on a diagnostic analyzer. Thus precise and accurate handling of small, e.g., micro liters, volumes of fluid is important to the performance of an assay being performed. The performance of available liquid metering systems, such as those on diagnostic analyzers, is sensitive to liquid properties, such as viscosity, in part because a higher viscosity liquid is more likely to adhere to the surface of an aspirating probe or the metering tip of such a probe, and also leads to the aspiration of less liquid than intended. The pressure signature or profile of the gas, e.g., air, between the pumping mechanism and the liquid during metering is a function of liquid viscosity. A typical pressure profile is shown in FIG. 1.
Known viscosity estimation techniques for metering systems, in known diagnostic analyzers generally work only for aspiration volumes larger than 35 μL. Such metering systems can be used on known diagnostic analyzers such as the Vitros 5600 and Vitros 3600 diagnostic analyzer sold by Ortho-Clinical Diagnostics, Inc. and other known analyzer systems because such analyzer typically measure volumes of liquid that are greater than 35 μL.
Thus, there is a need for a satisfactory estimation of liquid viscosity when small volumes of liquids, e.g., less than 35 μL and much less, are involved in a metering process, such as aspirating or dispensing a liquid.